1. Field of the Invention
This invention relates to an image processing system for measuring and controlling the alignment of various layers and for measuring critical dimensions of integrated circuits. The system permits alignment of masks and measurement of critical dimensions with accuracy greater than the resolution of the optical system employed in performing the alignment and measurements. In particular, this invention relates to a system by which Laplacian and Gaussian operations may be performed on pixel data in essentially real time for image processing, and to the correlation of the outputs of such operations for images.
2. Description of the Prior Art
In my prior work at Massachusetts Institute of Technology, I developed, in conjunction with others, a digital Gaussian convolver for image analysis which convolver included a serpentine memory for producing a column of 32 pixels simultaneously for each pixel input from a camera. A multiplier-accumulator then applied a one-dimensional 32-point Gaussian convolution to that column, following which a 32-element shift register stored the output of the multiplier-accumulator. A second multiplier-accumulator then applied a one-dimensional 32-point Gaussian convolution to the values in the shift register. The application of two of these Gaussian convolvers to the same image using slightly different sized Gaussian convolution functions in each convolver and taking the difference of the outputs achieved an approximation of the Laplacian of Gaussian convolution.
While this technique was satisfactory for some applications, it was undesirably slow and required a large number of expensive components; for example, at least 64 multiply-adds must be performed in each pixel clock period. In addition, this approach operated proportionally more slowly due to the serial multiply-accumulate steps in its design. Another disadvantage of this approach was that it could not be expanded beyond its 32-by-32 operator size without being completely redesigned.